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Apart from bandwidth constraints (uploading 10GB+ is not going to be fun), there are no constraint. It is just a matter of time before someone (including Apple) develops some kind of bindings to use Time Machine.

Alternatively, alot of people are already backing up their Macs to online services using services such as Amazon S3 (in conjunction with software such as JungleDisk and Mozy.

The big challenge is that end-users' available bandwidth has not kept pace with the amount of data being created locally.

Just to add in another competing back-up product, to the others that have been listed already, I use CrashPlan which does a nice job of de-duplicating data (it doesn't copy the whole file every time it is changed by a few bytes, nor does it have to transfer it again if it's renamed, or duplicated.)

CrashPlan offer a pay-for central backup hosting service (like the Mozy and JungleDisk solutions), or you can get your friends to install it and exchange back-ups (they're all encrypted at source, so they can't see what you've sent them.)

For a majority of connections, the initial upload (possible hundreds of gigabytes) is infeasible. Even the "unlimited" connections are most certainly not, and almost all have fair-usage policies which would hinder such transfers.

Also a service receiving hundreds of gigabytes of data from hundreds/thousands of customers seems like a fairly crappy business..

There are such online backup companies (there's loads, like Mozy and Backblaze), but I can't work out how they make enough money to pay for the storage/bandwidth/staff - offering "unlimited storage for just $5" does sound too good to be true, and seems like it is..

Backing up to something like Amazon S3 (or rsync.net), which charges based on the amount of data you store, seem more sane - but Mozy has been around for a while and does seem to be doing quite well